All-optical switching between waveguides in dense-wavelength-division multiplexed (hereinafter, “WDM”) networks using microring resonators is well known. However, such switching is limited by optical loss effects. For example, a microring resonator made of an electroabsorptive material coupled to two straight semiconductor side-channel waveguides has been proposed that could operate as a 2×2 crosspoint switch when the rings were switched from a low-absorption state to a high-absorption state. See, Soref et al. “Proposed. N-Wavelength M-Fiber WDM Crossconnect Switch Using Active Microring Resonators” IEEE Phontonics Technology Letters, Vol. 10, No. 8, (August 1998), which is hereby incorporated by reference in its entirety. As described by Soref et al., the rings could be made of layered III-V semiconductor heterostructure materials, and individual cross-points can be made of two-ring devices. Nonetheless, material loss, waveguide bending loss, and related fabrication-technology related losses, such as wall-roughness that occurs during waveguide etching, are known to limit optical switch performance.
Another challenge facing the implementation of integrated optical chip applications is the extremely small sizes required to make these waveguide devices commercially viable. To make microrings sufficiently small for intergrated optical chip applications, they require very large refractive index differences between the core material and the cladding material. Unfortunately, a large refractive index difference typically exhibits large scattering from sidewall imperfections, which causes large attenuation level. See, Little et al. “Microring Resonator Channel Dropping Filters,” Journal of Lightwave Technology, Vol. 15, No. 6 (June 1997) (hereinafter, “Little et al.”). If an appropriate optical material could be formulated for making a waveguide, coupling that waveguide to a microresonator could still be problematic. For example, to couple a conventional silica waveguide to a silica microsphere resonator, the waveguide typically is clad with materials of significantly lower refractive index to avoid leakage of radiation modes into the cladding. Because cladding materials having refractive indices that are less than silica (e.g., the material forming the microresonator) are not readily available, it is believed that basic waveguide coupling schemes have been frustrated. See, Laine et al. “Microsphere Resonator Mode Characterization by Pedestal Anti-Resonant Reflecting Waveguide Coupler” IEEE Photonics Technology Letters, Vol. 12, No. 8, at 1004 (August 2000), which is hereby incorporated by reference in its entirety. It is well established that in a waveguide structure, such as a microring resonator, aside from coupling losses, the total waveguide loss, which includes contributions from waveguide side walls, bends, and material losses, should be approximately equal to, or less than, 0.5 dB/cm in maganitude, and preferably less than 0.2 dB/cm. For a highly transparent optical medium to be used as the waveguide material, a fundamental requirement is that the medium exhibits little, or no, absorption and scattering losses. Intrinsic absorption losses commonly result from the presence of fundamental excitations that are electronic, vibrational, or coupled electronic-vibrational modes in origin. Further, the device operating wavelength of the microring resonator should remain largely different from the fundamental, or overtone, wavelengths for these excitations, especially in the case of the telecommunication wavelengths of 850, 1310, and 1550 nm located in the low loss optical window of a standard silica glass optical fiber, or waveguide. Material scattering losses occur when the signal wave encounters abrupt changes in refractive index of the otherwise homogeneous uniform optical medium. These discontinuities can result from the presence of composition inhomogenieties, crystallites, microporous structures, voids, fractures, stresses, faults, or even foreign impurities such as dust or other particulates.
In a waveguide structure comprised of a uniform square, or circular, waveguide cross-section, the waveguide material should exhibit little, or no, polarization dependence in signal propagation through the material. A potential source for polarization dependent behavior is the birefringence of the waveguide material. Birefringence is quantified by the difference in the refractive indexes for different polarization states for the propagating waveguide signal. The origin of material birefringence can be either intrinsic such as from the atomic structure or morphology of the material, or extrinsic such as from the effects of induced, or externally applied, force fields, or both. Composite materials are also well known, and generally comprise two or more materials each offering its own set of properties or characteristics. The two or more materials may be joined together to form a system that exhibits properties derived from each of the materials. A common form of a composite is one with a body of a first material (a matrix) with a second material distributed in the matrix.
One class of composite materials includes nanoparticles distributed within a host matrix material. Nanoparticles are particles of a material that have a size measured on a nanometer scale. Generally, nanoparticles are larger than a cluster (which might be only a few hundred atoms in some cases), but with a relatively large surface area-to-bulk volume ratio. While most nanoparticles have a size from about 10 nm to about 500 nm, the term nanoparticles can cover particles having sizes that fall outside of this range. For example, particles having a size as small as about 1 nm and as large as about 1×103 nm could still be considered nanoparticles. Nanoparticles can be made from a wide array of materials. Among these materials examples include metal, glass, ceramics, refractory materials, dielectric materials, carbon or graphite, natural and synthetic polymers including plastics and elastomers, dyes, ion, alloy, compound, composite, or complex of transition metal elements, rare-earth metal elements, group VA elements, semiconductors, alkaline earth metal elements, alkali metal elements, group IIIA elements, and group IVA elements
Further, the materials may be crystalline, amorphous, or mixtures, or combinations of such structures.
Moreover, nanoparticles themselves may be considered a nanoparticle matrix, which may comprise a wide array of materials, single elements, mixtures of elements, stoichiometric or non-stoichiometric compounds
The host matrix may be comprised of a random glassy matrix such as an inorganic glass, or organic polymer. Suitable inorganic glass hosts include but are not limited to doped and undoped silica such as aluminosilicate glasses, silica, germania-silica, lithium-alumina-silica, sulfide glasses, phosphate glasses, halide glasses, oxide glasses, and chalcogenide glasses. Organic polymers may include typical hydrocarbon polymers and halogenated polymers.
By introducing nanoparticles into the core of the waveguide structure, the absorption and scattering losses due to the nanoparticles may add to the waveguide propagation loss. In order to keep the waveguide propagation loss to a minimum, in addition to controlling the loss contribution from the waveguide host matrix, it is essential to control the absorption and scattering loss from the nanoparticles doped into the waveguide core.
For discrete nanoparticles that are approximately spherical in shape and doped into the host matrix, the scattering loss α, in dB per unit length, resulting from the presence of the particles is dependent on the particle diameter d, the refractive index ratio of the nanoparticles and the waveguide core m=npar/ncore, and the volume fraction of the nanoparticles in the host waveguide core Vp. The nanoparticle induced scattering loss can be calculated by:                               α          =                      1.692            ×                          10              3                        ⁢                                          (                                                                            m                      2                                        -                    1                                                                              m                      2                                        +                    2                                                  )                            2                        ⁢                                                            d                  3                                ⁢                                  V                  p                                                            λ                4                                                    ,                            (        1        )            where λ is the vacuum propagation wavelength of the light guided inside the waveguide. As an example, when m=2, Vp=10%, λ=1550 nm, d=10 nm, the calculated scattering loss α is 0.07 dB/cm. To fabricate a certain waveguide device with a set loss specification, and therefore a nanoparticle induced waveguide loss budget of α, the nanoparticle diameter d must satisfy the following relationship:                               d          <                                    (                              α                ⁢                                  1                                      1.692                    ×                                          10                      3                                                                      ⁢                                                      (                                                                                            m                          2                                                +                        2                                                                                              m                          2                                                -                        1                                                              )                                    2                                ⁢                                                      λ                    4                                                        V                    p                                                              )                                      1              /              3                                      ,                            (        2        )            where λ is the vacuum propagation wavelength of the light guided inside the waveguide, m=npar/ncore the refractive index ratio of the nanoparticles and the core, and Vp the volume fraction of the nanoparticles in the host waveguide core. For example, following Equation 2, with a nanoparticle loss budget of α=0.5 dB/cm, when m=2, Vp=10%, λ=1550 nm, the nanoparticle diameter d must be smaller than 19 nm. In general, the diameter of the nanoparticles must be smaller than 50 nm, and more preferably, 20 nm.
Nanocomposite materials including nanoparticles distributed within a host matrix material have been used in optical applications. For example, U.S. Pat. No. 5,777,433 (the '433 patent) discloses a light emitting diode (LED) that includes a packaging material including a plurality of nanoparticles distributed within a host matrix material. The nanoparticles increase the index of refraction of the host matrix material to create a packaging material that is more compatible with the relatively high refractive index of the LED chip disposed within the packaging material. Because the nanoparticles do not interact with light passing through the packaging material, the packaging material remains substantially transparent to the light emitted from the LED.
While the packaging material used in the '433 patent offers some advantages derived from the nanoparticles distributed within the host matrix material, the composite material of the '433 patent remains problematic. For example, the composite material of the '433 patent includes glass or ordinary hydrocarbon polymers, such as epoxy and plastics, as the host matrix material. While these materials may be suitable in certain applications, they limit the capabilities of the composite material in many other areas. For example, the host matrix materials of the '433 patent commonly exhibit high absorption losses.
Additionally, the method of the '433 patent for dealing with agglomeration of the nanoparticles within the host matrix material is inadequate for many composite material systems. Agglomeration is a significant problem when making composite materials that include nanoparticles distributed within a host matrix material. Because of the small size and great numbers of nanoparticles that may be distributed within a host matrix material, there is a large amount of interfacial surface area between the surfaces of the nanoparticles and the surrounding host matrix material. As a result, the nanoparticle/host-matrix material system operates to minimize this interfacial surface area, and corresponding surface energy, by combining the nanoparticles together to form larger particles. This process is known as agglomeration. Once the nanoparticles have agglomerated within a host matrix material, it is extremely difficult to separate the agglomerated particles back into individual nanoparticles.
Agglomeration of the nanoparticles within the host matrix material may result in a composite material that lacks a desired characteristic. Specifically, when nanoparticles agglomerate together, the larger particles formed may not behave in a similar way to the smaller nanoparticles. For example, while nanoparticles may be small enough to avoid scattering light within the composite material, agglomerated particles may be sufficiently large to cause scattering. As a result, a host matrix material may become substantially less transparent in the presence of such agglomerated particles.
To combat agglomeration, the composite material of the '433 patent includes an anti-flocculant coating disposed on the nanoparticles intended to inhibit agglomeration. Specifically, the '433 patent suggests using surfactant organic coatings to suppress agglomeration. These types of coatings, however, may be inadequate or ineffective especially when used with host matrix materials other than typical hydrocarbon polymers.
It would therefore be desirable to overcome one or more of the problems or disadvantages associated with the prior art.